Fabrication of ultra-thin wall bobbins

ABSTRACT

A SHORT LENGTH OF SHEET-LIKE B-STAGE POLYESTERS SUPPORTED BY A NYLON SUBSTRATE IS PARTIALLY WRAPPED AROUND A CENTER PORTION OF A LAMINATED POLE PIECE FOR A MAGNETIC READ-ORRECORD HEAD, PREFERABLY WITH THE B-STAGE POLYESTR CONTACTING THE POLE PIECE. THE ASSEMBLAGE IS PLACED IN A HEATED COMPRESSION MOLD. THE MOLD IS CLOSED TO FORM AN ANNULAR SPACE ABOUT THE CENTER PORTION OF THE POLE PIECE AND   CAUSES THE POLYESTER AND NYLON TO FLOW ABOUT THE POLE PIECE TO FILL THE SPACE, THEREBY FORMING A UNITARY ANNULAR BOBBIN WITH A WALL THICKNESS SELECTED IN THE RANGE FROM ABOUT 1 MIL TO 15 MILS. AFTER THE POLYESTER/NYLON CONGLOMERATE HAS CURED, THE POLE PIECE WITH THE BOBBIN IS REMOVED FROM THE MOLD.

IIIVENTUR AUGUSTUS B. 880" ATTORNEY u n n 1 H 0 n a. n u 0 a u I A. B. BROWN FABRIGA'I'IONjOF ULTRA-THIN WALL BUBBINS Filed March 4, 1969 May 4, 1971 United States Patent 3,577,506 FABRICATION 0F ULTRA-THIN WALL BOBBINS Augustus B. Brown, Longmout, Colo., assignor to International Business Machines Corporation, Armonk, N.Y. Filed Mar. 4, 1969, Ser. No. 804,173 Int. Cl. B29d 3/00 US. Cl. 264-272 3 Claims ABSTRACT OF THE DISCLOSURE A short length of sheet-like B-stage polyesters supported by a nylon substrate is partially wrapped around a center portion of a laminated pole piece for a magnetic read-orrecord head, preferably with the B-stage polyester contacting the pole piece. The assemblage is placed in a heated compression mold. The mold is closed to form an annular space about the center portion of the pole piece and causes the polyester and nylon to fiow about the pole piece to fill the space, thereby forming a unitary annular bobbin with a wall thickness selected in the range from about 1 mil to 15 mils. After the polyester/nylon conglomerate has cured, the pole piece with the bobbin is removed from the mold.

BACKGROUND OF THE INVENTION This invention relates to the in-place fabrication of insulating bobbins on an article and, more particularly to the in-plate fabrications of such bobbins having a microminiature thickness.

The dimensions of electrical apparatus, particularly in the data processing area, are continuously being reduced in size. This reduction in size has many advantages in that higher frequency operation is usually more easily attained, less power consumption, facilitates moving same, and reduces the cost of installation. Included in the microminiaturization of electrical equipments are the read/ record heads used with magnetic tapes systems, magnetic disk systems, magnetic drums and the like. Each head has a pair of pole pieces which may consist of laminations of high MU 80 material having a high magnetic permeability. A coil of wire is wound around one of the pole pieces for causing or sensing magnetic fluctuations within the pole piece. Such magnetic fluctuations are transferred between the head and a magnetic media via a gap between such pole pieces, as is well known.

The wire used to fabricate such a coil usually has a very small cross-sectional area, for example, less than 1 mil in diameter. To prevent shorting of the turns, such wire is insulated from the pole piece. Previously, tape had been wound around the extremely small center portion of pole piece (for example, a pole piece may have one diameter of 0.153" and a thickness of 0.038" and a total length of or less). Tape placed around the pole piece, such as a cellophane self-adhesive tape, can form an insulating layer; however, the edge of the tape may be sufficiently sharp and rigid to sever or cause a short between two adjacent small diameter wires. Accordingly, it is desired that the bobbin holding the wire have a smooth contour thereabouts. It is also desired that the thickness of the bobbin be kept to a minimum in order to maximize the inductive coupling between the coil of wire and the pole piece.

SUMMARY OF THE INVENTION It is an object of the present invention to provide inplace molding of an insulating bobbin having a microminiature thick webbing.

It is another object of this invention to provide a pole piece assembly having relatively large pole piece ends with a reduced center section having a unitary insulating bobbin thereabouts with micro-miniature dimensions.

According to this invention, a deformable article, such as a laminated pole piece, is nested in a compression molding die. A sheet-like plastic preform is placed in juxtaposition to the article. The mold is then closed with the spacing between the mold and the article being less than the thickness of the plastic preform. Applied heat permits the plastic preform to fiow throughout the cavity between the mold and the article to form a unitary annular bobbin about the article. Typical dimensions of the bobbin web in thickness is from less than one mil to not greater than 15 mils. Typical thickness of the preform is from two to three times that of the thickness of the ultimate bobbin web. The sheet-like plastic preform may have a nylon substrate with B-stage semicured polyester resin layer thereon. The polyester layer preferably faces the article around which the bobbin is to be formed. The extent of the preform may be less than that of the ultimate bobbin, but has a thickness greater than the ultimate thickness of the bobbin web.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an enlarged perspective view of a laminated pole piece having a preform disposed thereon in the center section thereof.

FIG. 2 is an enlarged cross-sectional view taken in the direction of the arrows along line 2-2 of FIG. 1.

FIG. 3 is an enlarged diagrammatic cross-sectional view of the FIG. 1 illustrated pole piece while in a molding machine of the compression molding type.

FIG. 4 is a diagrammatic cross-sectional showing of a pole in the FIG. 3 illustrated mold taken in the direction of the arrows along line 44 in FIG. 3.

FIG. 5 is a greatly enlarged perspective view of a pole piece having an in-place molded bobbin thereabout together with a few turns of wires forming a coil.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS Referring now more particularly to the appended drawings, like numerals indicate like parts and structural features in the various views. A pole piece 10 is formed of a large plurality of high MU laminations 11 and shaped as shown in FIG. 1. Pole piece 10 includes an enlarged gap-forming end portion 12 and an opposite enlarged low reluctance return portion 13. A magnetic head is completed by placing a low reluctance bar between end portions 12 and 13 with a shim being disposed immediately adjacent in 12 as at 14. Pole piece 10 also includes a reduced center or central section 15 which is to ultimately receive a coil of wire 16 (FIG. 4) thereabout. The coil of wire 16 consists of very small diameter wire, for example, having a diameter less than 1 mil. Such wire is quite subject to abrasion, breakage, and other forms of damage. If damaged, the coil could have its turns shorted by the lamina 11 which is usually formed of high MU 80 sheet material. Accordingly, an insulating bobbin is disposed around center section 15 to protect the coil from damage as well as to facilitate fabrication thereof.

For illustrative purposes, center section 15 has a thickness of 0.038", a width of 0.153", and a total length between the end portions 12 and 13 of about 0.325". For maximizing magnetic coupling, it is also necessary to keep the coil wound as closely as possible to center section 15.

According to the present invention, a short length of sheet-like plastic tape is selected and placed around center section 15. This sheet-like material 20 can be purchased in tape form with the tape being cut to length in accordance with the physical dimensions of center section 15. The tape 20 consists of two layers with an aggregate thickness of about 0.003" to form a bobbin with a web having a thickness of about one mil. A nylon (polyamide) carrier or substrate 21 having a thickness of less than one mil supporting thicker layer 22 of B-stage thermosetting polyester (cured or cross-linked upon being heated to a predetermined temperature). As shown in FIG. 2, the B-stage polyester is preferably placed against center section 15 to enhance adhesion of the ultimate bobbin to center section 15. It is understood that nylon layer 21 can be placed against center section 15 with successful results being obtained. During the later described compression molding operation, there apparently is a mixing of the nylon and the polyester during the curing of the polyester such that the two layers are no longer distinguishable.

The B-stage polyester is a known trade term for semicured polyester. That is, it is a polyester in which the crosslinking for a solid is partially completed. During the later described compression molding, the cross-linking is completed (i.e., the polyester is cured to its solid state). Without the nylon carrier (i.e., a high temperature plastic) the B-stage polyester could not be placed in sheet form. Using semi-cured or partially cross-linked polyester shortens the curing time to thereby reduce the probability of damage to pole piece 10.

After the short length of plastic tape 20 is placed over center section 15, the assemblage is disposed in the lower part 30 of a pair of mold parts 30 and 31, with the free ends of the plastic tape forcing upwardly. It was found that the free ends of the length of tape 20 tend to curl inwardly as shown in FIG. 2. The assemblage fits in the mold part 30 quite snugly, no limitation thereto intended. When mold parts 30 and 31 are closed, the pole piece is securely held therebetween. The two mold or die parts 30, 31 are operated between open and closed positions by press 32 operatively connected to the mold parts as indicated by dotted lines 33. Movable cavity sections 38 and 39 complete the mold assembly. Construction of a compression mold press and its relationship to die parts or molds is well known and is not further described for that reason.

When the die parts 30 and 31 are closed, as shown, the mating faces 34 and 35 thereof form a seal therebetween to contain the plasticized material during molding operations. A pair of locating pins 36 accurately align die parts 30 and 31, one with respect to the other. As illustrated in FIGS. 3 and 4, pole piece 10 is inserted in first cavity or nest 33 of die part 30 with the plastic sheet 20. After the die parts 30 and 31 have formed a seal between their faces, cavity sections 38 and 39 are forced toward one another by press 32 until there is a space of about one mil between the opposing faces 40 and 41 of the cavity sections 38 and 39, and pole piece 10. As cavity section 38 moves downwardly, lower surface 40 engages the two free ends of the length of tape 20, forcing same to move toward each other and toward center section 15. Prior to such movement, a heater 44 applies heat to the die parts for heating the plastic material to an easily flowable state. The pressure applied by the cavity sections 38 and 39 force the plastic sheet 20 to flow about the central section to form the annular plastic bobbin having web 45 and flanges 50. The movement of cavity sections 38, 39 toward each other is limited by engagement of the facing surfaces 40 and 41. Cavity sections 38 and 39 have recesses 46, 47, 48, and 49 to form flanges 50 in the bobbin. When surfaces 40 and 41 are engaged, recesses 47 and 48 form one annular recess while recesses 46 and 49 form a second annular recess, respectively, for forming flanges 50.

Several bobbins were fabricated on a like plurality of pole pieces such as above described. The temperatures of the mold parts was maintained between 290 and 300 F.

The time of curing was varied from three to five minutes. The molding pressure was maintained at 500 p.s.i. Variations within the above described temperature and time ranges resulted in successful formations of plastic bobbins around the center section 15 of the plurality pole pieces. The bobbin web 45 was about 1 mil thick with the end outwardly extending flanges 50 having a radial dimension of about 3 mils. While the 1 mil spacing over the /8" length of center section 15 would, at first glance, appear to severely limit the plastic flowing, no difiiculties were encountered in fabricating the bobbin in place using the above described sheet of plastic material in a compression mold. It is believed that the utilization of the sheet-like plastic facilitated the flowing of the plastic throughout the annular cavity between center section 15 and the cavity sections 38 and 39. Alternatively, the single length of tape 20 could be replaced by two shorter lengths of plastic tape (not shown) respectively placed above and below center section 15 as viewed in FIG. 4. That is, there would be one length of tape placed on surface 40 and the second on surface 41 at the cavity forming recess portion 41A. In the constructed samples, plastic tape 20 was purchased from American Cyanamid Co. as No. WE-4014 adhesive. The utilization of such a sheet-like plastic preform is believed to reduce the shear rate of the plastic flowing within the compression mold enabling the plastic to fill the micro-miniature cavity. If an excess of plastic is utilized in the preform (i.e., the volume of the plastic preform is greater than the volume of the mold cavity), then an aperture can be formed in one of the molds or die parts to permit the excess to flow out from the cavity. Such an aperture could have a diameter of about 1 mil. Such relief apertures are known in the molding art.

An advantage of using the polyester/nylon combination is that the mold can be opened while still warm. If pure nylon would be utilized, one would have to wait until the mold cools before moving the article. This requirement reduces the production rate of in-place molded bobbins. Further, the polyester is believed to have better adhesive properties and thereby bond to center section 15 for preventing any movement between the bobbin 45, 50 and pole piece 10.

It is also possible to use other molding techniques than compression molding. For example, open side injection molding could be used. After the plastic material is injected into the mold area, the mold would then be closed and the polyester cured as above described.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A process for in-place molding of unitary annular bobbins about a reduced-sized center section of a deformable magnetic article having enlarged end portions, the process being performable in a molding machine having opposed and facing movable die parts respectively having first and second cavity-forming movable portions with such movable portions forming a cavity of predetermined dimensions when the die parts and cavity forming portions are closed by the molding machine;

the improved process including the combination of the following steps:

disposing a deformable magnetic article having a reduced-sized center section on one of such die parts and adjacent said first cavity forming portion when said cavity forming portions are opened and aligning said center section with said cavity forming portions, said center section having a cross-sectional volume slightly smaller than said cavity and being disposed centrally for forming 6 a uniform annular space in said cavity when said time between about three to five minutes at a temperature cavity forming portions are closed; of between about 290 to 300 F. disposing a single sheet-like thermosetting type plastic preform in juxtaposition and partially References Ciied wrapped around said center section having free UNITED STATES PATENTS ends forming an acute angle adjacent one side of said center section, said preform having volumetric and uniform extent substantially that of said annular space such that flow length of said preform during molding is minimized; closing said mold die parts and simultaneously closing said cavity forming portions to form said 2,663,908 12/1953 Maier et al.

264--(Partia1 cure digest) 2,910,456 10/1959 deGooreynd et al.

264-(Partial cure digest) 2,912,418 11/1959 Johnson et a1.

264(Partial cure digest) cavity with an annular space being formed about 2943359 7/1960 Su.sslnan 264272X 2,975,487 3/1961 Williams, Jr. 264-272 said article in said cavity, said cavity forming 3 276 114 10/1966 B] k 2 52 portions compressing said preform causing it to auioc 7 3,354,542 11/1967 Mallia 264-272X flow evenly thereabout filling said annular space,

3,388,458 6/1968 Logan 264-272X curing said plastic into a unitary annular ring 3 458 611 7/1969 R b 264 275x around said center section; and 0 arts said center section, when disposed in said closed FOREIGN PATENTS mold portions, having a spacing from said cavity forming portions of not greater than 15 mils and 4061849 3/1934 Great f f 264272 being held in place by engagement by said die 964,349 7/ 1964 Great Brltaln 264272 parts, and said sheet-like preform being selected to originally having a thickness slightly greater ROBERT WHITE, Primary Examiner than Said p A. M. SOKAL, Assistant Examiner 2. The process of claim 1 wherein said sheet-like plastic preform consists of a nylon carrier supporting a layer US. Cl. X.R. of B-stage thermosetting polyester. 264-325 3. The process of claim 2 wherein said mold is closed for curing said sheet-like preform plastic for a period of 

